1. Field of the Invention
This invention relates to a method and apparatus for controlling the gradation of an ink jet printer apparatus configured for applying an electrical voltage to a electrostriction oscillator provided on a printing head for ejecting liquid ink for recording on a recording medium.
2. Description of the Related Art
In a color printer apparatus employing an ink jet type recording system, there are two primary methods for ejecting the ink: continuous ink ejection, in which the ink is continuously converted into liquid drops and only necessary liquid drops are sprayed onto a printing support material, and on-demand or intermittent ink ejection, in which the ink is formed into liquid drops for printing on a printing support material.
The on-demand type ink jet printer apparatus employs a printing head of an electromechanical conversion system in which a piezoelectric element is used as an electrostriction oscillator and the force of distortion of the piezoelectric element is utilized for varying the volume of an ink chamber for expelling the ink as liquid droplets via a nozzle. Alternatively, the on-demand type ink jet printer uses employs a printing head of a heating vaporization system, which is of the electro-mechanical conversion system, in which the ink is instantaneously heated and vaporized by a heating element annexed in the nozzle for expelling the ink via the nozzle under the pressure of generated air bubbles. The printing head of the electro-mechanical conversion system or the electrical heating conversion system is called a pressure pulse system printing head.
Specifically, the printing head of the electro-mechanical conversion system may be in the form of an oscillating cylinder type printing head in which a cylindrical electrostrictive oscillator is mounted on a lateral surface of a tube formed of glass or the like and an electrical voltage is impressed on the oscillator for causing changes in the ink chamber volume to expel ink particles. Alternatively, the printing head may be in the form of a planar plate oscillator type printing head in which a planar plate type electrostriction oscillator and an oscillator plate are bonded to each other to produce a bimorph to which an electrical voltage is applied to produce changes in the volume of an ink chamber for expelling the ink particles, or the printing head maybe in the form of a stem type printing head in which an ink supply conduit and a pressure chamber are separated as ink chambers and a bimorph is used for expelling ink particles.
For printing an image by the ink jet printer apparatus on a recording paper as a recording medium, liquid droplets of a liquid ink of a uniform size are expelled for reproducing the gradation using a two-gradation dithering method corresponding to the presence or absence of the ink liquid droplets. This dithering method resides in adding noise formed in accordance with a pre-set rule (dither) to input video signals for producing a bi-level signal using a pre-set threshold value. The dithering method may be classified into an independent decision method in which a threshold value independent of the input video signal is used for setting bi-level signals and a conditional decision method in which the threshold value is changed depending on the state of the input video signals. The independent decision method may further be classified into a random dithering method employing a random number uniformly distributing threshold values and a systematic dithering method employing periodic threshold values. The conditional decision method may further be classified into a mean error minimizing method for correcting the next pixel data value by a weighted mean value of errors of plural pixels, an error diffusion method of diffusing errors produced in a pixel to subsequent pixels, a mean value limitation method in which a threshold value is determined from a mean value of surrounding pixels for determining of a pixel under consideration, and a dynamic threshold method aimed at compressing the gradation picture.
In the above-described ink jet printer device, having the printing head of the electro-mechanical conversion system and designed to reproduce the gradation using the dithering method, the printed image has insufficient gradation because the image is printed with liquid droplets of liquid ink of unitary size by the dithering method designed for reproducing two gradation steps.
Moreover, when raising the gradation of the image, the image is lowered in effective resolution due to use of the dithering method.
In the ink jet printer device, having the thermo-electric conversion type printing head and designed to reproduce the gradation using the dithering method, the size of the ink liquid droplets corresponding to the data value to be printed, that is the dot diameter on the paper, is not changed, and hence the printed image has insufficient gradation because the principle of generation of the ink liquid droplets is dependent on the phenomenon of ink vaporization.
In JP Patent Kokoku Publication JP-B-6-39185 (1984), there is disclosed a simple dithering method in which the ink liquid droplet is varied in diameter responsive to the input digital value for reproducing multiple gradations. The method disclosed in this publication, however, requires gradation correction due to fluctuations in ink discharging characteristics and voltage to liquid droplet diameter characteristics of the electrostriction oscillator.